Integrating Metabolic Imaging with Metabolic Intervention - Unfolding the Mystery of Rare Isolated Pulmonary IgG4-related Disease Masquerading as Lung Tumor.

IgG4-related disease (IgG4-RD) is a systemic autoimmune fibroinflammatory condition in which IgG4-positive plasma cell infiltration occurs in various organs. Diagnosing IgG4-RD when it presents as a lung mass can be challenging, as its clinical and radiological characteristics resemble those of lung cancer, and its presentation is also not diagnostic. This case presents an elderly male, a former smoker, with a lung mass, initially suspected to be lung carcinoma due to heterogeneous enhancement and spiculated margins on Contrast enhanced computed tomography (CECT). The diagnosis remained uncertain post-computed tomography (CT)-guided biopsy. FFlurodeoxyglucose Positron Emission Tomography- Computed Tomography (FDG PET-CT) revealed a metabolically active lung mass with mediastinal lymphadenopathy. Metabolic PET-CT-guided biopsy confirmed IgG4-RD with IgG4-positive plasma cells. This prevented unnecessary invasive procedures and ensured timely and appropriate treatment. The patient showed symptomatic improvement with steroid therapy. This unique case emphasizes the importance of combining metabolic imaging with metabolic intervention to accurately detect rare, isolated pulmonary IgG4-RD, which can clinically and radiologically mimic lung carcinoma.